Last revised by Jeremy Jones on 20 May 2023

Rhombencephalosynapsis is a congenital abnormality of the cerebellum characterized by the absence of the vermis and continuity of the cerebellar hemispheres, dentate nuclei, and superior cerebellar peduncles. It either occurs as an isolated anomaly (rare) or as part of a wider cerebral malformation and it has variable degrees of neurological impairment.

The condition is very rare and thought to be sporadic. No familial cases have been identified 9.

Clinical presentation includes truncal and/or limb ataxia, abnormal eye movements, head stereotypies, delayed motor development, and other features determined by supratentorial abnormalities (see below) 10. Overall patients tend to die in childhood or early adult life 9.

Rhombencephalosynapsis has been seen to be associated with:

Embryologically rhombencephalosynapsis is now considered to result from a failure of vermian differentiation. The fused development of the cerebellar hemispheres is thought to result from an insult between the 28th and 44th day of gestation 6,8. The classic appearances is of a transversely oriented single lobed cerebellum.

Rhombencephalosynapsis was traditionally thought to be a complete fusion of cerebellar hemispheres. However, off late cases of partial fusion have also been reported 6,8.

Rhombencephalosynapsis is best seen on MRI brain, and is characterized by 1-4,7,8:

  • vermian abnormalities

    • absent anterior (rostral) vermis

    • deficient posterior (caudal) vermis

    • nodulus tends to be formed

  • fusion of the cerebellar hemispheres, with transversely oriented inferior cerebellar folia

  • fusion of dentate nuclei

  • fusion of the superior cerebellar peduncles

This usually results in an abnormally small posterior fossa, and an abnormally shaped fourth ventricle, which takes on a diamond (or key hole) shape 7,8,10.

Additional frequently associated abnormalities include:

  • fusion of the thalami

  • fusion of inferior colliculi

  • absent septum pellucidum

  • ventriculomegaly: often from aqueductal stenosis

  • sutural synostosis

  • other supratentorial abnormalities 8

Variable clinical progression ranging from early death to varying degrees of cerebellar dysfunction and neurodevelopmental delay. Some patients may reach adulthood 3.

It was first reported by Heinrich Obersteiner (1847-1922), an Austrian neurologist, in 1914 3.

Imaging differential considerations include:

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